The prior art, FIG. 1 of Israel Patent 87071 describes an electronic warning system composed of two main portions: a central control unit 2 and a line 4 in a fence. The central control unit 2 includes a timer 6 governing the operation of a transmitter 8 electrically connected to the line 4 and of a counter 10 driven by an oscillator 12. A receiver 14 also connected to the fence 4 and arranged in circuit for actuating an alarming unit 16 and for activating the counter 10. The latter operates a time-to-distance translator 18, which translates the signal received by the receiver into distances measured from any reference point and displays the same on a suitable display unit 20.
The line 4, stretchable along a practical distance of up to about 1000 meters from each side of the central control unit 2, is composed of a pair of wires 22 interconnected at their ends by a resistor 24 and of a plurality of sensors 26, e.g., inertial sensors. The sensors 26 can be connected to the wires 22 in series, in parallel, or in a combined series and parallel fashion. The sensors 26 are spaced-apart from each other at typical distances of between 3 to 5 meters. Satisfactory detection results were obtained with the usage of sensors detecting and responding to vibrations or to changes in volume, operating in the ultrasonic or infrared frequency ranges. The resistor 24 is chosen to possess resistance, which is substantially identical to the characteristic resistance of the fence.
The teachings of the above-described Israel Patent are incorporated herein by reference.
Experience with this and other warning systems has shown that the rate of false alarms caused by wind, rain or hail, animals, birds and the like, is high, thus imposing an unnecessary burden on the guards.